Crystal rejector circuit



April 18, 1933. w. s. BARDEN CRYSTAL REJECTOR CIRCUIT Filed Jan. lILO, 1931 5 afrit/an WOL N E D R n B of m n ms o wm NN Nu A lum I/ W V.. B N w A. R A5 PR tu YM ma ww n F MOULA 770A/ FREQUENCY Patented Apr. 1.8, 11933 'unirse STATES PATENTa oFFlcE WILLAM S. BARDEN, O F STAPLETON, YORK, ASSIGNOR TO RADIO CORPORATION 0F AMERICA, ACORPORATIO1\T 0F DELAWARE CRYSTAL R13-moron CIRCUIT Application ied January 10, 1931. Serial No.i507,876.

ily present invention relates to electrical selective circuits, and more particularly to resonant circuits utilizing piezo-electric ar.- rangements for discriminating between de sired and 'undesired frequencies.

Quartz crystals have come into extensive use as substantially constant frequency sources for oscillators, on account-of peculiar characteristics which depend upon the inter-relation of mechanical and electrical properties of these crystals. lArnong these properties, perhaps the most.interesting,is that involving mechanical resonance with its associated counterpart in electrical resonance. As with most physical bodies the sharpness of resonance is exceedingly greatwhich has lead to many suggestions proposing its embodiment in radio receiving oircuits for accomplishing results which purely electrical arrangements have failed to do.

The quart-Z crystal lends itself particularly to use in selective circuits where it may have special utility due to its unique property of extremely great frequency discrimination. A problem exists in the present broadcasting situation which may apparently be solved by the use of frequency discriminating piezo-electric circuits. This problem involves the heterodyne interference which persists under the present l0 kilocycle channel separation assignment, and is caused by stations ydrifting from their proper frequencies. In most of these cases, however, which have been studied this annoyance at any one receiving point is created by only one interfering carrier, even though several stations in this country may be located on the same channel.

The zoning system employed at the prescnt day gives some freedom because of geographical separation. However, it can be readily shown that a heterodyne situation exists in different localities as shown in Fig. l. In this latter figure, an average heterodyne situation is graphically represented in which the two circular areas will be the .service areas of the two stations A and B on the same wave length, while the shaded areas C represent the heterodyning areas of the undesired transmitters in the service area of the local station. lf the carrier frequencies of these two stations A and B have drifted apart by a few hundred cycles, there is no known possible method of eliminating the beat note between them byl electrical, tuned circuits, without a considerable sacrifice in tone quality. Yet, this is the most annoying type of heterodyne interference.

Since the actual strength of the interference is usually relatively weak and the side bands therefore inappreeiable, but because of its very persistence is objectionable, it appears possible to employ the piezo-electric crystal as a sharply tuned rejector circuit to suppress the undesired carrier. Such a circuit would have little, or no, noticeable effect on ythe desired signal even though the two carriers were separated by only 200 cycles.

Accordingly, it is one of the main objects of my present invention to provide a method of, and means for, operating a resonant Circuit, adapted for use in connection with radio signalling, in such a manner that the circuit f discriminates between modulated radio frequencies differing only by a few hundred cycles, and as low as two hundred cycles.

Another important object of the invention is to provide a method of receiving voice modulated radio signals from a broadcast station without heterodyne interference from other stations of similar carrier frequency, which method consists in employing a piezoelectric means in the receiver as a sharply tuned rejector circuit to suppress the undesired, interfering carrier.

Another object of the invention is to provide a superheterodyne type of receiver having a sharply tuned crystal rejector circuit disposed between the first and second detectors, the crystal being resonant to the intermediate frequency employed, and the receiver being adjustable so as to place the crystal dip at the center of the resonance curve in order to suppress the interference properly, while preserving the desired programs, regardless of whether the interfering carrier is above or below the desired carrier frequency.

Still other objects of the invention are to improve the selectivity efficiency of resonant circuits, and more particularly to provide a. superheterodyne circuit which is free of undesirable heterodyne whistles caused by an interfering station which is relatively weak in strength, modulation, or both, and which is between 200 and 5000 cycles 0E the desired signal frequency.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invent-ion itself, however, as to both its organization and method of operation will best be understood by reference to the" following description taken in connection with thev drawing in which l have indicated diagrammatically several circuit organizations` whereby my invention may be carried intoeffect.

In the drawing, l

Fig. l is a graphic representation of an average heterodyne situation,

Fig.`2 is a diagrammatic representationof a circuit embodying the invention,

Fig. 3 is a resonance curve of the circuit shown in Fig. 2,

' Fig. 4l graphically acteristics of the circuit in Fig. 2,

Fig. 5 is a diagrammatic representation of a superheterodyne circuit embodying the invention,

Fig. 6 graphically shows the resonance curve of the'circuit in Fig. 5. Y y

Referring to the accompanying drawing in which like characters of reference indicate the shows the fidelity charsame parts in the dierent figures, there is shown in Fig. 2 an arrangement for eliminating the effect of the undesirable condition graphically represented in Fig. l. As pointed out heretofore, l shows the average heterodyne situation in which the circles surrounding the points A and B represent the service .areas of the two stations (situated at the points) on the same assigned wave length. The shaded areas @represent the heterodyning areas created by the undesired transmitter in the service area of the desired local station.

As long as the carrier frequencies of stations A .and B remain the same, receivers situated in either area C will not be affected by heterodyne effects. However, should either of the stations, or both, drift apart in carrier frequency by a few hundred cycles, then there will immediately be observed a heterodyne whistle in either area C, which eect is produced by the beat note between the two dierent carrier frequencies. Further, there is no known possible method of eliminating this beat note by electrical, tuned circuits, without .a considerable sacrifice in tone quality.

Assume now that the circuit shown in Fig. 2xis a. part of a receiver disposed in either C area. The receiver may be of any type,vthe amplifier and detector being shown in conventional form to simplify the description.

The input circuit of the detector, which includes a conventional electron discharge tube, comprises a tunable circuit consisting of an inductance coil l and a variable condenser 2. Thus, by varying the condenser, and as is Well known this condensermay be uni-controlled with variable tuning condensers in the radio frequency electron discharge tube amplifier stages, the desired modulated carrier from the local station, say A, is tuned in and the program thereof heard.

If the carrier of station B drifts away from the common frequency, the beat note is heard with .the program from A. Since the actual strength of the interference from B is usually relatively weak and the intensity of the side bands therefore inapp-reciable, but, because its very persistance is objectionable, it is possible to employ a piezo-electric means, such as a quartz crystal 3, as a sharply tuned rejector circuit, in shunt with the condenser 2, to suplpress the undesired carrier frequency from and thereby eliminate the beat note.

The crystal 3 is conventionally represented, Vit being, understood that it is constructed in av manner Well known to those skilled in the art, and that alternating electromotive forces impressed on its opposite fiat sides cause the crystal to expand and contract in accordance with the applied electrical impulses. Structurally, the crystal is usually mounted between metal electrodes which function to apply the electromotive force to the crystal. Only in the immediate vicinity of the natural period of the crystal does the crystal vibrate violently dueto the applied force, with a resulting appreciable in-phase current iiow in the applied electrode connections 3.

Thus, it becomes obvious that the quartz crystal shunt circuit possesses an electrical characteristic whose equivalent may be representedby a resonant, electrical circuit including inductance, capacitance, and resistance and sharply tuned to the frequency represent-ingthe natural period of the crystal.

The crystal 3, in Fig. 2, has a natural period corresponding to the undesired carrier frequency. Thus, in Fig. 2, the variable condenser 2 is adjusted to tune the circuit l, 2 to the carrier frequency of the station A, while the crystal 3 would be resonant to the carrier frequency of station B. v

It is to be observed .that the crystal rej ector circuit has little, or no, noticeable effect on the desired modulated carrier frequency, even though the carrier frequencies of stations A and B are separated by as little as 200 cycles. y In Fig. 3 Iy havey shown a resonance curve which illustrates the Y electrical functioning of the circuit diagrammatically represented 'f in Fig. 2. It will be noted that there is a sharp dip, termed hereinafter a crystal dip, at or about the peak of the resonance curve. Assuming that the electrical circuit l, 2 in Fig. 2 is tuned to a carrier frequency F, and

the crystal 3 is tuned to the undesired carrier F1, then the carriers F and F1 would be spaced and disposed along the resonance curve in Fig. 3 as shown. It will be observed from the resonance curve in Fig. 3 that certain high frequency components ofthe side bands of the carrier F are eliminated simultaneously with the suppression of the undesired carrier F1. It has been determined that such elimination of such components does not have any appreciable effect on the fidelity characteristic of the desired signals.

As sho-wn in Fig. 4, a circuit as used in Fig. 2, and Whose resonance curve is shown in Fig. 3, results in an interesting fidelity characteristic. It is shown by Fig. 4 that the crystal dip is not only exceedingly sharp, but never rccedes more than half way to zero. In other words, the voltage'attenuation at that particulai modulating frequency is never more than 50%. This is explained by the fact that the dip in Fig. 8 merely suppresses that one side band component which has the same frequency as the interfering carrier, but has no effect on the other side band component which constitutes one half .of the audio response. Such a crevice in the audio response curve is not aurally noticeable, while the suppression of the continuous howl is practically complete, and therefore the reception becomes practically as good as though the interfering signal were non-existent.

lt will be understood that the circuit in Fig. 2 is based on the lassumption that the resonant frequency of the crystal 3 can be readily changed to suit different conditions. rlhis, of course, is not the case. Crystals are constructed to one definite desired resonance by a slow and delicate cutting process requiring eXtreme precision. Obviously, if the frequency of crystal resonance cannot bechanged, and only one crystal is to be used for all reception conditions, it is necessary to employ a circuit which permits a change in the frequencies of received carriers so that an interfering carrier may be deliberately shifted to coincide with the crystal dip. Superheterodyne arrangements offer the only known solution.

Thus, in Fig. 5 there is shown, in diagrammatic fashion, a superheterodyne receiver circuit embodying the usual grounded antenna circuit A, which antenna circuit is coupled by any well known means, as at M, to a radio frequency amplifier arrangement, the latter preferably comprising a plurality of electron discharge tubes having tunable input circuits. 'lhe amplifier is conventionally represented as shown, it being understood that the resonant circuit which designates the tuning means of the radio frequency amplifier includes the variable condenser 2, the latter being adjusted by means of the usual manual knob 4. The output of the amplifier is coupled, by any suitable means as at M1, to the input of the first detector, the latter comprising an electron discharge tube arranged in .any well known manner for mixing the oscillat-ions from the local oscillator and the amplified radio frequency signals to produce the desired intermediate frequency.

The local oscillations are produced in a conventional local oscillator, the input circuit of the latter being designated by the reference numeral l and being tunable by a Variable condenser 2', the latter being unicontrolled, as shown lby dotted lines, from .the knob 4. The local oscillations produced in the local oscillator are impressed, through a -circuit 5, upon the firstdetector in order to produce the intermediate frequency in the output circuit of the first detector tube. The intermediate frequency energy is amplified in the usual intermediate frequency amplifier which 'is coupled to the output of the'lirst detector, as at M2, the intermediate 'frequency amplifier comprising one or more stages of amplification including one or Ymore electron discharge tubes conventionally represented as shown in Fig. 5, The amplified intermediate frequency energy is then impressed upon the input circuit of a second detector, the latter including an electron discharge tube conventionally represented.

The output of the second detector may then be further amplified by anyy Well known type of audio amplifier or utilized in any other manner as by head phones, loud speaker or the like. Ther input circuit of the second detector is coupled to the intermediate -frequency amplifier output circuit, as at-Mg, the input circuit including an inductance coil 7 and a capacity 8, the resonant circuit .7 8 being usually lixedly tuned to the intermediate frequency utilized. There is 'arranged -i-n shunt with the capacity 8 a crystal rejector circuit, as shown in Fig. 2, the circuit comprising the crystal 3 dispos-ed between metal electrodes, the metal electrodes being connected, as at 3', to the opposite plates of the capacity 8. A source of biasing potential 9 is shown in series` with the inductance coil 7 it being understood that the biasing-source 9 may be omitted if desired, but is shown in korder to clearly represent the detection action of the second detector. The natural period of the crystal 3 is so chosen that it is equal to th-e intermediate frequency employed. In other words, the resonant circuit 7, 8 and the piezo-electric crystal 3 both are resonant to the same frequency, viz; the intermediate frequency.

In Fig. 6 there is shown a resonance curve produced by the circuit shown in Fig. 5.- It will be seen that the' electrical circuits are so tuned as to cause the crystal dip to fall directly at the electrical resonance peak. That is to say, when the knob 4: is manipulated so as to adjust the signal and local oscillator circuits l to' receive lthe desired program, and

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a whistle is heard, then'the lrnob lis adjusted until such whistle disappears. At the point of disappearance of the whistle, it will be understood that the signal circuit 1 is tuned to receive the undesired carrier frequency, and the local oscillator has been adjusted to produce the intermediate frequency .with such undesired carrier frequency. This results in the crystal dip at the electrical resonant peak, since the crystal circuit is al ways resonant to the intermediate frequency.

It will therefore be seen that by the present arrangement a single crystal can be employed as a rejector circuit to eliminate an undesired carrier frequency which causes a beat note, no matter in which portion of the broadcast range the undesired carrier is located, by simply adjusting the signal circuit l to the undesired carrier that is to say, tothe point where the heterodyne whistlel disappears) and producing the intermediate frequency by beating the local oscillations with the Aundesired carrier. Y

A resistance l0 of the order of 100,000 ohms is placed across the electrical circuit 7, 8 to effect a sufficient broadening of admittance so as to allow the'desired carrier and associated side bands to be received with substantially equal facility within the resonance curve when the undesired carrier has been set at the crystal dip. z It is desirable to place the crystal dip at vthe centerlof the resonance curve in order to suppress the interference properly, whilepr'eservin-g the desired program, regardless of whether the interfering carrier is above or below the frequency of the desired carrier.v

To illustrate the aforegoing statement, suppose the interfering carrier exists 500 cycles below the frequency of the desired carrier, it ,being understood that this state of affairs exists after conversion to the intermediate frequency. A slight manipulation of a Vernier on the superheterodyne oscillator places the interfering carrier at the crystal dip in Fig. 6, while the desired carrier falls in the region B. In the reverse instance, with the interference atV higher frequency, the desired carrier is located vin the region A. Of course, a drift inthe undesiredrcarrier frequency could be followed readily by a slight readjustment of an oscillator Vernier in order to maintain the interference carrier at the crystal dip frequency point.

lVhen the desired'carrier is transferred to the crystal dip frequency point by adjustment of the oscillator Vernier, the quality'of reception is immediately ruined, the distorted gibberish remaining being caused by the side bands of the desired carrier beating with each other, with the slight carrier remaining and with the interfering carrier. Meanwhile, the heterodyne whistle has vanished-because it was created by the desired and undesired carriers, one of which is now almost wholly crystal.

lacking. Accordingly, it will be understood that'disposing either carrier in the dip will `eliminate the whistle, butL in one case (undesired carrier coinciding with the dip) the quality is undisturbed in accordance with the rier and side bands, but the result is not seri-k ous cross-talk" in the ordinary sense, being a weak hash punctuated occasionally with peak breakovers. As stated heretofore `the arrangement described is useful, with regard to present Aconditions in the broadcast field, to eliminate heterodyne whistles caused by interfering stations which are relatively weak in strength, modulation, or both, and which are vbetween 200 and 5,000 cycles olf of the desired signal frequency. It should be noted that if the frequency difference is less than about 200 cycles, the crystal rejector circuit is not as effective in vview of the fact that limitations exist even in the use of the vention into effect, it will be apparent to one slilledv in the art that my invention is by no means limited tothe particular organizations shown and described, but that many modifications may be made without departing from thescope of my invention as set forth in the appended claims.

llVhat I claim is:

l. In a signal receiving system, means for producing local oscillations, a signal circuit tunable to a desired carrier frequency, means for heterodyning the desired frequency and local oscillations to produce a beat frequency,

While I have indicated andldescribed several systems for'carrying my inm means for detecting the beat frequency and a piezo-electric rejector path between said heterodyning means and said detecting means resonant to said beat frequency, and means for tuning vsaid signal circuit to an undesired interfering carrier frequency slightly different from the desired carrier frequency when heterodyning occurs between the desired carrier frequency and said undesiredcarrier frequency.

2. Aniethod of operating-a superheterodyne r, ceiver which consists in producing local osci lations, collecting desired modulated carrier energy, combining' the modulated energy and the local oscillations to produce an intermediate frequency energy, transmitting the intermediate yenergy through a filter sharply resonant to the intermediate frequency, detecting the filtered intermediate frequency energy, and combining undesired carrier energy of a frequency slightly different from said desired carrier with said local oscillations to produce said intermediate frequency to eliminate heterodyning between such undesired carrier energy and desired modulated carrier energy.

3. In combination in a radio receiver, a tunable signal circuit, a tunable local oscillator, means for producing an intermediate frequency, a detector circuit coupled to said intermediate frequency means and including an input circuit consisting of an electrical path resonant to said intermediate frequency, a piezo-electric path resonant to the intermediate frequency in shunt with said electrical path, and a resistor in shunt with said piezo-electric path of a magnitude sufficient to effect a broadening of the admittance of said electrical path.

4. A method of receiving modulated carrier energy of a desired frequency without interference from carrier energy of a frequency differing from the desired frequency by an amount of the order of 200 to 5,000 cycles which consists in amplifying the combined desired and undesired carrier energies in an amplifier resonant to said undesired carrier frequency, beating the amplified carrier energies with local oscillations to produce beat energy, transmitting the resultant beat energy through a path sharply resonant to a frequency equal to the difference between the local oscillation frequency and said undesired carrier frequency, and detecting the transmitted energy.

5. A method of receiving intelligence modulated carrier energy of a desired frequency without interference from intelligence modulated carrier energy of a frequency differing from the desired frequency by an amount r less than 5,000 cycles which consists in amplifying the combined desired and undesired modulated carrier energies in an amplifier resonant to said undesired carrier frequency, beating the amplified modulated carrier energies with local oscillations of a frequency sufficient to produce energy of an intermediate frequency, transmitting the resultant intermediate energy through a path sharply resonant to said intermediate frequency, and detecting the transmitted energy.

6. A method of eliminating heterodyne interference, in a superheterodyne receiver, between broadcast stations having carriers of the same frequency, the interference being caused by a slight variation in the frequency of one of said carriers, which consists in resonating the receiver input to the carrier frequency which has varied, and ltering the intermediate frequency energy of the receiver through a piezoelectric path resonant to WILLIAM S. BARDEN. 

